This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
A marine (subsea) drilling riser may be a conduit employed during drilling that provides an extension of a subsea oil well to a sea-surface drilling facility. For example, one end of the subsea drilling riser may interface with a subsea blowout preventer (BOP) at the wellhead, and the other end of the subsea drilling riser may interface with a floating drilling vessel at the sea level surface. Subsea drilling risers generally include a low-pressure main tube or conduit having a relatively large diameter and that conveys drilling materials, and in some cases, production fluids, between the drilling vessel and the well. The subsea drilling riser also has external auxiliary lines which may include a high pressure choke and lines for circulating fluids to the BOP. The auxiliary lines may also include power and control lines for the BOP. The design and operation of subsea drilling risers may be complex, and reliability may involve engineering analysis.
The marine (subsea) drilling riser may be tensioned for stability. A marine riser tensioner located on the drilling platform may provide a substantially constant tension force to maintain the stability of the riser in the offshore environment. The level of tension may be related to the weight of the riser equipment, the buoyancy of the riser, the forces from waves and currents, the weight of the internal fluids, and an allowance for equipment failures. To reduce the amount of tension to maintain stability of the riser, conventional buoyancy modules may be added to the riser joints to make the risers neutrally buoyant when submerged. An international standard ISO 13624-1:2009 covers design, selection, operation and maintenance of marine riser systems for floating drilling operations. The standard serves as a reference for designers, for those who select system components, and for those who use and maintain this equipment. The standard generally relies on basic engineering principles and the accumulated experience of offshore operators, contractors, and manufacturers. However, marine (subsea) drilling risers may also be designed, constructed, and maintained based on other standards or to general practice without reference to a particular standard.
The art employs concentrated buoyancy elements, at the top of the lower riser section, in the form of buoyancy cans, tanks, or inflatable bladders (such as in U.S. Pat. Nos. 4,234,047, 5,046,896, 5,657,823, and 5,676,209) or combinations of concentrated and dispersed buoyancy elements such as in U.S. Patent Publication No. 2009/0044950. Spread buoyancy employing actively controllable buoyancy via a plurality of gas-filled chambers is found in U.S. Pat. No. 4,646,840.
The stationkeeping capacity may be the capability of the surface drilling vessel to maintain its position relative to a reference point or to the subsea wellhead being drilled. In many subsea drilling scenarios, environmental conditions such as approaching storms or ice, have potential to impose loads on the drilling vessel in excess of the stationkeeping capacity. In such scenarios and other examples, the drilling operation may be stopped, the well shut-in, and the station keeping system disconnected from its anchors. The stationkeeping system including the drilling vessel may be relocated to an area or region on the sea remote from adverse environmental conditions. If so, a reduction in the time to suspend drilling operations, shut-in the well, and disconnect the stationkeeping system from the well may be beneficial. A significant time component may be the time to retrieve the drilling riser, particularly in deeper water depths. There is an on-going desire to improve shut down of drilling operations and the disconnection of the drilling vessel in impending adverse environmental conditions.